dual head VGA Cards

I have an hp pavillion p6235cn
and i have put an ATI PCI vga card on the PCI slot and installed its drivers. I then removed it and
PUT an ATI PCI-Express on the PCI-E slot, installed its drivers and then PUT back all cards and booted the machine.

PROBLEM:
on the BIOS , only the PCI Slot that is set as default works (and you can see its drivers on device manager without the other drivers for other cards). The others are disabled.

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I don't think you can.
The BIOS makes the PCI-Express card the default display adaptor.
It will overide onboard and any other PCI display adaptor on the system.

What are you trying to acheive?
If you want to run two monitors you need a PCI-Express video card with twin outputs.
Some come with one analog and one digital (DVI) ports.
Most of the latest cards have two DVI ports and one HDMI port.

Does the motherboard support SLI or X-Fire?
If it does you can install two PCI-express cards with dual DVI ports.
This will give you the potential to connect four monitors.

If you want to run more than four monitors you will have to invest in video card that supports multi display outlets and these are very expensive.

Yes this is correct, the bios will either do this automatically upon detection of a dedicated expansion card in this case a PCI-express video card or a PCI vidoe card that will overide the onboard display adaptor.

Or as you say you can manually select but you cannot use both.
It's one or the other.
Once a card is installed the onboard video is disabled in bios.

Was this the fifth outlet you were hoping to use as I see you mentined five outputs.
What operating system do you have on the computer?
How old is the HP?

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